Vapor discharge lamp with cooling means for portion of electrode



Nov. 19, 1968 w. E. THOURET 3,412,275

VAPOR DISCHARGE LAMP WITH COOLING MEANS FOR PORTION OF ELECTRODE INVENTOR WOLFGANG E. THOURET BY %r% l'i'diii Filed Oct. 12, 1966 III ATTORNEYS United States Patent Office Patented Nov. 19, 1968 ABSTRACT OF THE DISCLOSURE A compact arc discharge lamp in which one or both of the electrodes are fluid cooled internally. A portion of one or both electrodes spaced from the tip on which the arc is produced has a reduced thickness to increase the operating temperature of that portion of the electrode and the overall operating temperature of the lamp. The reduced thickness portion of an electrode is preferably insulated from the fluid by a member to prevent the temperature of this portion from dropping. The production of the elevated temperature in the lamp permits the use of a metal fill, and/ or metal additive, and/ or halogen to produce a halogen transport reaction.

This invention relates to electric discharge lamps and more particularly to a novel electric discharge lamp of the compact arc type having a rare gas or metal vapor filling in which a desired temperature distribution is maintained.

High pressure compact arc type (i.e. arc electrodes placed relatively closely together rather than at the ends of an elongated tube) discharge lamps with either rare gas or metal vapor filling, have recently found increased usage in optical equipment. Typical applications include the use of these lamps as light sources for search lights, projectors, and devices that simulate the suns radiation in outer space for the purpose of testing space vehicles or their components. Lamps of this type are very suitable for such equipment because they provide a stable source of radiation with a very high brightness and good luminous efficacy. They also operate cleanly without adjustment or maintenance for hundreds, or thousands, of hours.

Because of their many advantages, high pressure compact arc type lamp are increasingly replacing carbon arc type lamps which were previously used in many of the aforementionul applications. Since carbon arc lamps are available or can be built for higher input wattage ratings up to 20, 30 'or even 100 kilowatts, it is desirable to provide compact arc discharge lamps having similar high wattage ratings. This demand has been partly met through development of rare gas (xenon) high pressure compact arc lamps having input ratings in the -13 kilowatt range.

For lamps with a rare gas filling, such as xenon, an input wattage rating exists above which economic and reliable lamp designs cannot be obtained without using forced cooling of the electrodes. This limit presently seems to lie somewhere between 10 and kilowatts, depending upon the particular conditions of the application for which the lamp is to be used. For mercury, mercury additive and other metal vapor types of compact are type lamps, the input wattage limit without using forced cooling appears to be somewhat higher because these lamps require smaller currents, electrodes and seals for the same wattage, than a rare gas filled lamp.

The use of forced cooling increases the wattage ratings of compact arc type lamps to a considerable degree. For example, high brightness xenon lamps in the 20 to 30 kilowatt input range have been developed using liquidcooled electrodes and seals. Succh lamps operate economically and reliably, because due to the liquid cooling, quartz-to-metal seals of relatively simple construction can be used. The electrodes of these fluid-cooled lamps are capable of carrying currents up to 600 or 800 amperes and they have practically unlimited life because they operate at only slightly elevated temperatures.

The lives of lamps with uncooled seals and electrdes are usually terminated by seal failure, because the seals have to be operated at a temperature of several hundred degree Centigrade. In general it can be said that liquid cooled electrodes are highly advantageous for large wattage compact arc lamps because they allow both the use of smaller bulbs for the same wattage as an uncooled lamp and the use of simplified high current seals. However, even when liquid cooled electrodes are used, the bulb dimensions required for long reliable lamp life designs become quite large when the total lamp input exceeds 20 kilowatts. It has been found, for example, that in a compact arc lamp with liquid cooled electrodes the quartz bulb cannot be loaded to more than approximately 45 watts/cm? or 280 watts/sq. in. If loaded higher, the operating temperature of the quartz bulb increases into the range in which structural changes occur fast enough in fused quartz to reduce the mechanical strength of the envelope during the expected useful life of the lamp. The term structural changes is used to mean all changes that lead to a decrease in tensile strength, for example, local recrystallization. The latter can take place spontaneously but it generally is caused by small foreign particles deposited on the inner surface of the lamp envelope which serve as crystallization centers. Such foreign particles are always present within the lamp through evaporation of electrode materials, especially tungsten, from the tips of the electrodes which operate at a very high temperature at those points where they are in direct contact with the arc plasma. In addition to initiating local recrystallization within the envelope, the evaporated tungsten gradually increases the amount of heat absorbed by the bulb wall. Thus, the bulb wall temperature rises and the structural changes are accelerated.

It is known that a halogen additive used in a lamp provides a transport reaction which prevents electrode particles from depositing on the bulb inner wall. This reaction prevents both the formation of the crystallization centers and blackening of the bulb wall. It is also known that the presence of a certain minimum partial pressure of a halogen additive protects the lamp bulb from chemical attack by ionizable meal vapors that are added to the basic rare gas and/or ionizable metal (e.g. mercury) filling for the purpose of improving the brightness, luminous efficacy and spectral energy distribution of the light output. While the use of these metal vapor additives is desirable, practically all of them that are useful as an additive to or instead of the mercury vapor usually used in a lamp, attack the quartz bulb violently. However, if a sufficiently high partial pressure of a halogen is present, this halogen combines with the metal atoms of the vapor additive to form a metal halogenide before the vapor additive reaches the bulb wall or at the wall itself before a damaging chemical attack takes place.

Heretofore, the design of compact arc type lamps with liquid cooled electrodes was limited to the use of a rare gas filling only, without the addition of metal vapors and/ or a halogen or halogen compound additive. Further, compact are type lamps with a metal vapor fill, with or without a metal additive and a halogen or halogen compound additive, could not be successfully operated with liquid cooling.

In the prior art liquid cooled, compact arc type lamp designs, essential parts of the inner volume of these lamps,

mainly the seal areas, are cooled to such low temperatures that metal additives cannot be effectively used. This occurs because the liquid cooling prevents the metal additives from reaching a sufliciently high vapor pressure at which a halogen cycle reaction can take place between the metal additive and a halogen additive. This is true for both lamps with a rare gas fill or a metal vapor fill. Further, the liquid cooling in prior art lamp designs prevented the lamp envelope from reaching an overall temperature high enough to permit the use of a metal or metal vapor as either the main fill or as an additive for improving the lamp operation.

In general, a temperature of at least 300 centigrade must be maintained over the entire inner lamp volume for the halogen cycle reaction to occur. A halogen cycle reaction also requires that all inner surfaces of the lamp be either of tungsten or quarts and glass. This is so because most other metals form halogen compounds that are stable and have lower vapor pressures than the cor responding tungsten halogenide. Therefore, the presence of these other metals within a lamp disturbs the tungstenhalogen cycle. Also, mercury and practically all other substances that can be useful in compact arc lamps as additives, or by themselves, as the main fill, require at least 500 Centigrade as a minimum overall temperature within the lamp. This high an overall temperature previously has not been obtainable in liquid cooled compact arc type lamps.

According to the present invention a novel liquidcooled compact arc type lamp is provided in which a predetermined minimum temperature, for example, at or above 500 centigrade, is maintained everywhere within the vacuum sealed portions of the lamp. This permits the addition of one or more of several metal vapors, such as sodium, thallium, indium, scandium, thorium, molybdenum, lanthanum, dysprosium, in addition to a basic rare gas filling. It also permits the use of a basic metal vapor filling, such as mercury vapor, with or without one of the above metal additives. In lamps of the present invention using the metal vapor additives, thet brightness and luminous eflicacy of the arc discharge is considerably increased to more than twice the value obtainable in pure rare gas or mercury arc lamps with the same wattage. The invention also permits the use of halogen or halogen compound additive, in either a gas or metal vapor filled lamp. The halogen additive operates efliciently at the elevated lamp temperatures obtained by the present invention, with either the electrode material and/ or the additive metals to reduce the wattage, and consequently the envelope size needed for production of a given light output.

It is therefore an object of the present invention to provide a compact arc discharge lamp utilizing a halogen cycl transport reaction.

A further object is to provide a liquid cooled compact arc discharge lamp using a halogen additive.

Another object is to provide a compact arc discharge lamp with liquid cooled electrodes in which a certain minimum desired temperature is maintained throughout the interior of the lamp bulb and those portions which are sealed.

An additional object is to provide a liquid cooled compact arc discharge lamp utilizing a metal vapor as its main fill.

Other objects and advantages of the present invention will become more apparent upon reference to the following specification and annexed drawings which shows a view, taken partly in section, of a lamp made in accordance with the present invention.

FIG. 1 shows a high pressure, rare gas or metal vapor filled, compact arc lamp 1 which has a spherical, or nearly spherical bulb with two integral generally cylindrical, extensions 12 and 13. The bulb 10 and its extensions are preferably made of a suitable refractory, transparent or translucent material such as quartz. A liquid cooled, hollow anode electrode 14 having a generally curved end 16 and a liquid cooled hollow cathode electrode having a generally pointed end 17 opposing the anode end 16, are mounted within the respective extensions 12 and 13. Current is supplied to each electrode through a suitable electrical cable 18. Each electrode also has a conduit inlet 20 to supply cooling fluid under pressure from a source (not shown) and to remove the heated fluid. The details of conduits 20 are not shown but each may be of any suitable configuration, for example, a pair of concentric tubes.

The sealing arrangement and construction of both electrodes is similar so the same reference numerals are used where applicable. Each electrode is formed with a ring 22 which has a feathered edge 23. The rings 22 are preferably made of a suitable high temperature material, such as tungsten, and they are either an integral part of the electrode, which in this case would mean that the electrode is also of tungsten, or they are brazed with a vacuum tight seal to the electrode body. The feathered edge 23 of each ring 22 is sealed to the respective bulb extension 12 or 13 by means of so-called graded seals 26. These seals comprise a plurality of different glasses with graded expansion coeflicients. The purpose of this is for bridging the difference between the expansion coefflcient of the ring material (e.g. tungsten) and the bulb material (e.g. quartz).

Tungsten is preferably used as the metal directly sealed to the highest expansion glass of the graded seal, the glass most remote from the end of the corresponding electrode, because most other metals require the use of glass types with softening point temperatures which are too low. As has been explained above, the lamp designed according to this invention can maintain a minimum temperature on the order of 500 centigrade within the entire inner lamp volume. This means that the entire graded seal area 26 must operate at or above this temperature Only hard glass type that require direct sealing to tungsten can meet this condition in a reliable manner.

Each electrode 14, 15 has a cooling tube 28 mounted therein which communicates with the inlet side of a respective conduit 20. A hollow chamber 29 is formed at the end of each electrode body around the outlet end of a tube 28. Cooling fluid, such as water is discharged from the outlet end of a tube 28 into a chamber 29 and streams directly against the metallic inner surfaces of the active ends 16 and 17 of the electrodes. This fluid directly cools the portions of the electrodes which are in the high temperature plasma arc. The heated fluid passes back to the outlet side of its conduit 20 through an annular passage 30 which surrounds the tube 28.

It should be apparent that the forced fluid cooling of the. lamp of FIG. 1, lowers the operating temperature of the electrodes, especially the anode. It has been found that up to of the wattage input can be removed as electrode loss energy through the cooling. In lamps with uncooled electrodes this loss energy heats the electrodes, in DC. operated lamps especially the anode, to a very high temperature and the heat is removed only through radiation. This radiated heat energy has to pass through the quartz envelope which partly absorbs it and thus is heated to higher operating temperatures. To keep the quartz bulb operating temperature within limits wherein its structure is not impaired the bulb dimensions have to be increased. As a consequence, lamp costs rise rapidly and reliability decreases with growing wattage. These disadvantages are not encountered in the fluid cooled lamp of the present invention.

As indicated above, lamps according to the present invention can have the following fills:

(1) Rare gas and halogen or halogen compound additive preferably used with tungsten electrodes.

(2) Rare gas plus metal or meal vapor additive and preferably a halogen or halogen compound additive.

(3) Metal or metal vapor, for example, mercury hal0- gen or halogen compound additive also preferably used.

(4) Metal or meal vapor, plus a metal or metal vapor additive and preferably also a halogen or halogen compound additive.

When using the above fills with a halogen or halogen compound additive, the overall temperature of the bulb must be maintained above 300 C. It must be maintained above 500 C. when using a metal or a metal vapor as a main fill or as an additive. If these temperatures are not maintained, the halogen and metal additives will not be effective for their intended purposes. In prior art lamps, it has been found that the cooling effect of the liquid is so strong that it reduces the temperature of portions of the lamp to below that needed for the halogen transport reaction and for a metal fill or metal additive to be effected. Such a result of a low temperature, for example, permits interior metal portions of the lamp to be adversely attacked by the metal fill or metal additives.

A predetermined minimum temperature is maintained in the lamp in the manner described below. Various points on the lamp for one electrode are marked with the letters A through F, to aid in the explanation. As shown, the wall of the body of each electrode has a reduced thickness area 32 over the entire length of the inner seal area, that is between the points marked B and E. This reduction can be, for example, to less than half the thickness of the outer electrode end marked A and the operating ends 16 and 17. Through this reduction in wall thickness the high operating current, between 300 to 800 amperes in a typical lamp, flowing through the electrode body has a strong heating effect over the reduced thickness area 32 which substantially completely balances the cooling effect of the liquid flow over the same area.

Each electrode body is also provided with a heat insulating tube 36 over the full length of the seal area, that is, from point B to beyond point B. The insulating tube 36 comprises, for example, ceramic material or fused quartz. The tube fits closely into the reduced thickness portion 32 of each electrode body and prevents direct contact between the cooling liquid and the electrode body of the entire area that has to be kept at the high temperature, for example 500 C. The insulating tube 36 is kept in place after insertion into each electrode bodys bore by a spring ring 38.

The outer surface of each electrode body inwardly of the points C and E is preferably coated with a layer of hard material such as glass 40. This portion of the electrode is generally the part that assumes temperatures, for example, in the order of from 550 to 850 C. during lamp operation. Within this temperature range tungsten surfaces are subject to attack by any halogen contained in the lamp atmosphere. The hard glass coating 40 prevents such attack. Above the mentioned temperature range no protection is required because the halogens usable in the lamp types under discussion do not form stable tungsten compounds. Tungsten is not attacked appreciably by the halogen below the mentioned temperature range.

A chart of the operating temperature of one typical lamp made in accordance with the present invention is given below with the various points as indicated in FIGURE 1:

The electrode bodies 14 and15 are preferably made entirely of tungsten to meet the requirements of the halogen cycle reactions explained below. The electrode bodies also can be made of other heat resistant metals, for example, molybdenum. Where a halogen cycle is to be used, all molybdenum surfaces exposed to the inner lamp atmosphere are densely coated with a layer of tungsten. Such tungsten coating can be made electrolytically or by any other suitable process.

The bulb 10 preferably has a halogen or halogen compound fill, preferably one having iodine or bromine. When maintained above a certain minimum concentration the halogen protects the envelope which is preferably of quartz. Through a halogen-tungsten halogenide transport reaction, evaporated tungsten particles from the electrodes are prevented from being deposited onto the inner bulb wall. If they have been deposited there, they are quickly removed by the same type of reaction. This means that no recrystallization centers are formed and that structural changes in the quartz are either prevented or .at least considerably delayed so that a higher bulb operating temperature and a higher bulb surface loading becomes practical, that is, the size of the bulb can be reduced. This also means that the optical absorption characteristic of the envelope remains essentially unchanged since there is no blackening of the bulb wall. Also, the bulb operating temperature does not increase substantially during lamp life.

When a metal vapor is used as a fill or an additive, the halogen also protects the wall of the bulb by reacting with the metal to form a halogenide.

As can be seen, the portion of the lamp between points C and F maintains a temperature of over 500 C. It is here where a tungsten-halogen and/or a halogen metal reaction takes place. This elevated temperature pennits both reactions to be efficiently utilized.

While a preferred embodiment of the invention has been described above, it will be understood that it is illustrative only, and the invention is to be limited solely by the appended claims.

What is claimed is:

1. A compact arc type discharge lamp comprising an outer envelope of refractory material, a pair of electrode bodies with closed ends to which electrical current is to be applied mounted within said bulb with the closed ends of said electrode bodies being oppositely disposed to produce an arc therebetween, means for fluid cooling at least one of said electrode bodies, and means for reducing the cooling effect of the cooling fluid on a portion of said electrode body spaced from the end of the electrode on which the arc is present so that said portion will have an elevated temperature.

2. A lamp according to claim 1 in which the fluid cooled electrode body has an interior bore in which fluid circulates.

3. A lamp according to claim 2 wherein said fluid cooling effect reducing means comprises a reduced thickness portion formed on said body and surrounding a portion of said bore for increasing the temperature of that portion of the body having the reduced thickness.

4. A lamp according to claim 2 wherein said fluid effect reducing means comprises a sleeve of heat insulating material within said bore to prevent the cooling fluid from contacting the inner surface of .a portion of the body.

5. A lamp according to claim 3 wherein said fluid effect reducing means further comprises a sleeve of insulating material located adjacent the reduced thickness portion of the electrode body to prevent the cooling fluid from contacting the inner surface of the electrode body.

6. A lamp according to claim 1 further comprising a quantity of a halogen material within said envelope, at least one of said electrodes having tungsten thereon, the lamp envelope temperature being elevated to a level suificient to permit a tungsten halogen transport reaction to occur.

7. A lamp according to claim 1 further comprising quantities of an ionizable metal material and a halogen material within said envelope, the lamp envelope temperature being elevated to a level sufficient to permit a combination reaction to occur between the metal and halogen.

8. A lamp according to claim 6 further comprising a material on a portion of at least one of said electrode bodies to prevent attack thereof by the halogen material.

9. A lamp according to claim 6 further comprising a quantity of metal material within said envelope, the lamp envelope temperature being elevated to a level sufiicient to permit a combination reaction to occur between the metal and halogen. t

10. A compact arc type discharge lamp comprising an outer envelope of refractory material, a pair of electrode bodies to which electrical current is to be applied mounted within said bulb with the ends of said electrode bodies being oppositely disposed to produce an arc there between, means for fluid cooling at least one of said electrode bodies to a predetermined temperature, an ioniz'able metal within said envelope, and means for maintaining at least a portion of said electrode body at a sufficiently higher temperature in order to vaporize the metal. 7

11. A lamp according to claim 10 wherein said lastnamed means comprises a portion of said electrode body formed so that said portion will have an elevated temperature.

12. A lamp according to claim 11 wherein the fluid cooled electrode body has an interior bore in which fluid circulates and wherein said electrode body is formed with a reduced thickness portion surrounding a portion of said bore for increasing the temperature of that portion of the body having the reduced thickness.

References Cited UNITED STATES PATENTS JAMES W. LAWRENCE, Primary Examiner. 

